Don’t know about the lakes but there is the Mawddach Trail (old railway line) just a ride/walk over from Barmouth along the Mawddach River to Dolgellau past the RSPB centre at Penmaeneuchaf (sp?). Basic facilities yet lovely views from campsite at Arthog (Garth y Fog) just over the lane from the trail. Lots of beach on hand. Cader Idris. Fairbourne mini/kids train long the beach. Cregennen Lakes, oh, and the George III bar which serves very good ale and food looking out over the Estuary.
Am watching this thread with interest as we’re looking for similar trails for (hopefully) later in the year.
Thinking about it – Garth y Fog is very basic for family camping, just a field, toilets, shower with coin meter,
There is also http://www.graigwen.co.uk/snowdonia-camp-site.php have a look there.
http://www.mawddachtrail.co.uk/mawddach-trail.html